The quality of crude oil is getting worse as the increasingly higher exploitation quantity. The current crude oil contains a high amount of heavy metals, sulfur, nitrogen, gum, asphaltene and acid value, and an increasingly higher density and viscosity. The price difference between inferior crude oil and high quality crude oil are becoming larger as the shortage of the resource of petroleum oil. There is more concern for the process which could treat inferior crude oil. It is a challenge for conventional fluid catalytic cracking (FCC) process to treat inferior crude oil to maximize the yield of the light oil.
Generally, there are three types of processing technologies for the heavy oil. The first type is hydrogenation technology which includes hydrotreating and hydrofining process. The second type is decarburization technology which includes solvent deasphalting, delayed coking and heavy oil catalytic cracking process. The third type is aromatics extraction technology. Inferior heavy oil could be converted to low boiling point compound using these technologies whereby the hydrogen to carbon ratio is improved. When the inferior heavy oil is processed by a decarburization technology, the decarburization technology is greatly influenced by the high amount of sulfur, nitrogen, heavy metal, aromatics, gum and asphaltene contained in the inferior heavy oil, so the yield of liquid product is low and the quality of the product is worse. For delayed coking, removal ratio of impurities is high, but coke yield is almost 1.5 times higher than carbon residue of the feedstock, and another problem is how to take use of the solid coke. The hydrogenation technology could remedy defects of decarburization technology, wherein liquid product yield is higher and has good quality, but the investment is relatively large. The armomatics extraction technology's investment is lower, and it can not only achieve good effects in the heavy oil processing, but also obtain aromatics as byproduct which is an important chemical engineering raw material.
CN1448483A discloses a combined process of hydrotreating technology and decarburization technology. Firstly, residual oil is processed in thermal cracking reactor under mild conditions, then the thermal cracking product and slurry of FCC are subjected to solvent deasphalting, and the solvent deasphalting oil is hydrogenated in the presence of hydrogenation catalyst and hydrogen. The advantage of this process is that severity of residual oil hydroteating unit decreases and hydrogenation catalyst's life is prolonged. The yield and quality of liquid product are improved, but deasphalted oil is difficult to be utilized.
CN1844325A discloses a combined process of hydrotreating technology and decarburization technology. Inferior heavy oil is processed with solvent deasphalting and delayed coking, then the solvent deasphalted oil and coker gas oil is hydrotreated as feedstock for heavy oil hydrogenation units. Hydrotreating condition is mild and quality of feedstock is improved, so the hydrotreating unit operating cycle period is prolonged. The combined process can provide good quality feedstock for the downstream FCC unit, but the process flow is complex and the liquid yield is low.
CN1382776A discloses a combined process of residual oil hydrotreating technology and heavy oil catalytic cracking technology. Residual oil, slurry distillate, FCC heavy cycle oil and optional distillate oil are introduced to hydrotreating unit as feedstock, which are subjected to a hydrogenation reaction in the present of hydrotreating catalyst and hydrogen. After gasoline and diesel oils are evaporated from the oils resulting from the reaction, hydrogenated residual and optional vacuum gas oil are introduced to FCC unit, which carry out catalytic cracking reaction. The resultant heavy cycle oil and slurry distillate from FCC unit are introduced to residual hydrotreating unit. Slurry and heavy cycle oil can be converted to high quality fuel oil with this combined process, so the yield of gasoline and diesel increases. The advantage of hydrotreated heavy oil introduced to FCC unit is the higher liquid yield and product quality. The disadvantage of the process consists in a severe hydrotreating condition, a high operating pressure and temperature, a low space velocity, a short operating cycle period, and a high investment, because the heavy oils have high density, viscosity and high amount of heavy metal, gum and asphaltene. Generally, quality of hydrotreated residue which will be provided to FCC is fluctuating from operating initial stage to operating end stage of the hydroteating unit, and the fluctuating feedstock quality will bring unfavorable effects to FCC unit. The composition of the feedstock of hydroteating unit is extremely complex, which contains not only sulfur, nitrogen, and metals, but also alkane, cycloalkane and aromatics. Alkane can be easily cracked to small hydrocarbons and even dry gas under hydrotreating conditions, resulting in an ineffective use of the heavy oil resource. There are 8%˜10 wt % unconverted heavy oil when the residue oil is processed in FCC unit, and thus it cause a low utilization efficiency of the heavy oil. Although the unconverted heavy oil can be introduced to hydrotreating unit, the improvement of unconverted heavy oil quality is limited, since the quality of the unconverted heavy oil and residual is greatly different and the heavy oil has a low content of hydrogen.
CN1746265A discloses a catalytic cracking process for inferior feedstock. Light diesel fraction product is cycled back to FCC unit and heavy oil is extracted by solvent. Heavy aromatics obtained from extraction are useful chemical materials. Raffinate oil is cycled back to FCC unit. The heavy oil problem is solved in some extent, but final boiling points of light diesel and heavy diesel need to be not less than 300° C. and 450° C. respectively. The light diesel is cycled back to FCC unit as feedstock and the heavy diesel is introduced into extraction unit. The raffinate of heavy diesel is introduced into FCC unit as feedstock. As a result, although the oil slurry content is decreased to some degree, it is still undesirably high. Moreover, there is no diesel product and the dry gas yield is relatively high.
CN1766059A discloses a method to treat inferior heavy oil or residual oil. Heavy oil or residual oil are introduced to a solvent extraction unit, and the resultant raffinate oil is introduced to fixed bed hydrotreating unit which produce hydrogenation tail oil. The hydrogenation tail oil is introduced to FCC unit which produces slurry and other products. A part of or all slurry and raffinate oil are introduced to suspended bed hyrotreating unit, in which the products are separated to produce light fractions and unconverted oil which is extracted in solvent extraction unit. The process have some advantages in treating heavy oil which combines catalytic cracking technology, extraction technology and hydrotreating technology, however, this process has a complex flow and the liquid yield is low.
Yield of high acid value and high calcium crude oil is increasingly exploited with the development of exploitation technology. Calcium compounds in crude oil are mainly non-porphyrins which are only dissolvable in petroleum fraction and hardly removed from crude oil by conventional desalination method. If acid value of crude oil exceeds 0.5 mgKOH/g, the conventional crude distillation unit almost could not treat this high acid value oil which corrodes the processing equipment. To this end, CN1827744A discloses a method to treat high acid value crude oil. Pretreated high acid value crude oil having a total acid value greater than 0.5 mg KOH/g is preheated and then introduced to FCC unit to contact with the catalyst to carry out catalytic cracking under the FCC reaction conditions, and then the resultant product vapors are separated from the catalyst and fed to a subsequent separation system. The spent catalyst is stripped and regenerated to recycle. This method has a good industry practicability, good deacidification and low operation cost, but both dry gas and coke yield are high.
For long time, ordinary technicians in catalytic filed hold a point that conversion of heavy oil in catalytic cracking is the higher the better. To the contrary, after creative thinking and experiments, the inventors surprisingly find that target products become less and less but dry gas and coke yield increases greatly when the conversion of heavy oil exceeds a certain degree.
In order to effectively utilize inferior heavy oil and meet the increasing market requirements of high quality fuel oil, it is necessary to develop a process to convert inferior heavy oil to a great amount of light and clean fuel oil.